We are in the process of investigating the role of adjunct elective postoperative radiotherapy for locally advanced resectable rectal and colonic cancers. From October 1972 to December 1976, sixty patients with rectal and colon cancers at high risk of local recurrences (B2, C1, C2) received postoperative radiotherapy. Patients with low lesions (rectal and recto-sigmoid) received 4500-5000 rads T.D. to the pelvis and paraortic lymph nodes by an inverted T field. Patients with lesions above the mid-sigmoid received treatment to the whole abdomen by the moving technique. Only 8% of patients developed local recurrences. 3% of patients with low lesions had local recurrence and 13% of patients with colonic tumors recurred locally. Only one patient with a colonic lesion had a fatal radiation injury to the small bowel. We propose to continue the present protocol for patients with rectal and low-sigmoid colonic cancer. However, for patients with tumors located above the mid-sigmoid, we propose to randomize patients between a postoperative whole abdomen irradiation arm and postoperative chemotherapy arm using 5 FU and Methyl-CCNU. Large field irradiation did not appear to alter in vivo and in vivo parameters of cell mediated immunity. However, there is suggestion that radiotherapy may induce the formation of antinuclear antibodies. We intend to continue our investigation of the effect of large field irradiation on the cell mediated and humorally mediated immune systems.